1. Field of the Invention
The present invention relates to an electrostatic coating apparatus and more specifically relates to a technique of checking the grounded state of a coating object.
2. Description of the Related Art
In the case of electrostatic coating, at least a specific area (coating surface) of a coating object where a coating material is to be applied needs the electrical conductivity. When a coating object having high insulation property, such as a resin component, is subject to electrostatic coating, the electrical conductivity is given to the coating object generally by mixing a conductive material such as carbon black with the constituent material of the coating object or by forming a conductive covering (conductive primer) on the surface of the coating object. During electrostatic coating, the coating object (coating surface) is grounded.
A difference in conductive performance between the respective areas of the coating object (coating surface) causes a variation in amount of application of the coating material, which may result in the non-uniform thickness of the coating layer or the uneven color of the coating layer. Unreliable grounding connection of the coating object (coating surface) causes the charge to accumulate on the coating object (coating surface) during electrostatic coating, which may result in generation of a spark between the coating object and a jig or the like located in the periphery of the coating object. It is accordingly determined whether the grounded state of the coating object is good or bad, prior to electrostatic coating.
A conventionally known technique of checking the grounded state of a coating object brings a specified terminal into contact with the coating object (coating surface) and measures the resistance value of the coating object. This method may, however, damage the coating object by the contact of the terminal. Accordingly, a proposed technique applies the charge to the coating object to charge the coating object and subsequently measures the surface potential on the coating object. This enables the grounded state of the coating object to be checked without being in contact with the coating object (see, for example, Japanese Laid-Open Patent Application No. 2005-58998A). Another proposed technique employs a spray gun used for electrostatic coating to apply the charge to the coating object and measure the surface potential. This does not need any special device for checking the grounded state and thereby tries to shorten the production line and reduce the equipment cost (see, for example, Japanese Laid-Open Patent Application No. 2012-71224A).